The large dark spot (top center) and its smaller companion (top right) on Neptune, as seen by the Hubble Space Telescope in 2020. The large dark spot – discovered in 2018 – unexpectedly changed direction and headed back on the road he had come. The smaller storm is thought to be a fragment of the larger storm. Image by NASA / ESA / STScI / MH Wong (University of California, Berkeley) / LA Sromovsky / PM Fry (University of Wisconsin-Madison).
Like its giant brothers Jupiter, Saturn and Uranus, Neptune has massive storms in its turbulent atmosphere, which can be seen in the most powerful telescopes on Earth. Scientists have been watching one of Neptune’s large places, first seen by the Hubble Space Telescope in September 2018. This time, the storm did something unforeseen. He changed direction, saving himself from the expected disintegration, at least for now. The surprising development was announced by Hubble astronomers on December 15, 2020 and presented at the fall meeting of the American Geophysical Union on the same day.
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This storm in Neptune is wider than the Atlantic Ocean at 7,403 km. It is the fourth dark storm on Neptune to be seen by Hubble since 1993. Previously, two other similar storms, including Neptune’s Great Dark Spot, had been seen up close by the Voyager 2 spacecraft in 1989, during Neptune’s historic flight. Unfortunately, they disappeared before Hubble took the stage in 1990 and could take a look.
Here is the smaller dark spot, which – according to scientists – is probably a fragment that broke the larger dark spot. Image by NASA / ESA / STScI / MH Wong (University of California, Berkeley) / LA Sromovsky / PM Fry (University of Wisconsin-Madison).
The change of direction of this new storm was not the only unusual thing. He also presented a notable lack of smaller, bright, feathered clouds that had often been seen above Neptune’s dark spots. According to scientists, those clouds disappeared when the dark spot stopped moving south. Bright clouds are composed of methane ice crystals that form when gases are deflected upward over dark spots and freeze into crystals.
The large dark spot also had a smaller companion – nicknamed “dark spot jr”. – first observed in January 2020, which astronomers believe could have come out of the main storm. This smaller place has finally disappeared. As Michael Wong of the University of California, Berkeley, explained:
We are pleased with these observations, as this smaller dark fragment is part of the process of breaking the dark spot. This is a process that has never been noticed. I’ve seen other dark spots disappear and disappear, but I’ve never seen anything disturbing, even if it’s predicted in computer simulations.
The new larger storm began its life in the northern hemisphere of Neptune and then began to gradually move south to the southern hemisphere of the planet. As can be seen in the observations of other such storms, from here they begin to disintegrate. But not this time. Instead, it was observed that the storm slowed down and changed direction by August 2020, moving back north from where it had come. This was the first time such behavior had been observed in 30 years of Hubble observations.
Michael Wong of the University of California, Berkeley, lead author of the new paper on the dark spots of Neptune. Image via the University of California, Berkeley.
Interestingly, the smaller dark spot appeared around the same time that the larger storm reversed course and began to return in the direction it had come from. This newer point was about 6,276 km long and appeared on the side of the larger point facing Neptune’s equator. This was the area where some computer simulations suggested that a larger place interruption might occur, but the calendar does not seem to fit. Wong said:
When I first saw the small place, I thought the largest was interrupted. I didn’t think a whirlwind was forming, because the little one is further to the equator. So it is in this unstable region. But we cannot prove that the two are related. It remains a complete mystery.
It was also in January [2020] that the dark whirlwind stopped moving and began to move north again. Perhaps throwing that fragment away was enough to prevent him from moving to the equator.
Neptune’s storms are similar to those of other gas giants and ice giants (Neptune is an ice giant) and can grow in size to easily empower the largest hurricanes or cyclones on Earth, becoming as large as The earth itself. Unlike terrestrial hurricanes, these are high-pressure systems that form at mid-latitudes before migrating closer to the equator. In the northern hemisphere, they rotate clockwise due to the Coriolis Effect. But this effect weakens as the storms approach the equator and the storms eventually disintegrate.
This is the general scenario that unfolds as the Neptune storms develop and then eventually break up. But in this case, the storm did not reach the “killing zone” beyond the equator. As Wong noted:
It was really interesting to see him acting the way he should and then suddenly he stops and turns back. It was surprising.
The Great Dark Spot on Neptune as seen by Voyager 2 in 1989. The white clouds of feathers above it are composed of methane ice crystals. Image via NASA / JPL / Wikipedia.
It is not yet known if the dark spot jr. they also survived, but scientists continue to analyze Hubble data. The new observations also provide valuable clues as to how Neptune’s atmosphere generally behaves.
Hubble has played a central role in observing these spots on Neptune, especially since no other spacecraft has visited the planet since Voyager 2. According to Amy Simon of NASA’s Goddard Space Flight Center:
I wouldn’t know anything about these last dark spots if it weren’t for Hubble. We can now watch the great storm for years and watch its full life cycle. If we didn’t have Hubble, then we might think that the Great Dark Spot seen by Voyager in 1989 is still there on Neptune, just like Jupiter’s Great Red Spot. And, we wouldn’t have known about the other four places Hubble discovered.
Until a new mission is (hopefully) finally launched, telescopes like Hubble will continue to observe the mysterious dark spots of Neptune.
Bottom line: A large, dark storm on Neptune unexpectedly changed direction, saving itself from impending destruction in the process. You may have produced a smaller pet storm.
Source: Neptune NDS-2018: The dark vortex that would not die
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